1. Field of the Invention
The present invention relates to an apparatus for measuring the flow velocity or the flow rate by detecting the number of vortexes that are formed. More specifically, the invention relates to an apparatus for measuring the flow velocity by detecting the number of Von Karman's vortex streets or Strouhl vortexes by utilizing heating wires.
2. Description of the Prior Art
A conventional device of this type has been disclosed in Japanese Utility Model Publication No. 14448/1973. According to this device, heating wires are running through the holes formed in poles that constitute a vortex generator, and the number of vortexes are detected relying upon the heating wires. With the device in which bare heating wires are disposed in the fluid to detect analog change in temperature of the heating wires, however, the sensitivity for detecting the change of vortexes undergoes variation depending upon the change in the flow velocity, and deteriorates frequency characteristics.
Further, in the case when a pair of bare heating wires are provided at the back of the vortex generator, the two heating wires being so controlled as to maintain constant temperatures respectively, in order to detect vortex signals relying upon the difference in the control signals of the heating wires, difference develops in the detected signals depending upon the characteristics of the operational amplifiers employed for the heating wires, and makes it difficult to stably detect the signals.
Moreover, variance in the operational amplifiers often renders the device inoperative. In addition, if the temperature of the fluid to be measured changes, resistance of the heating wires changes, too, and the detection sensitivity varies.